Are there materials which absorb the light from low-pressure sodium lamps and reemit at a different visible frequency? Sodium lamps can be very bright (at low energy consumption) so some way getting colors other than yellow would be nice. Probably only orange and red colors on the assumption that fluorescence cannot increase frequency.
Astronomers might become unhappy because they can't just filter out the narrow frequencies of sodium, though I don't see red light becoming used a lot for outdoor lighting. LEDs are pretty efficient at producing red light. (Astronomers are already unhappy with the white light LEDs (which rely on fluorescence) becoming prevalent in street lighting.)
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